A Drop of Rain

There is also evidence that the bombardment of protons from the sun may create small amounts of new water when the charged nuclei of hydrogen combine with free-floating oxygen in the upper atmosphere. Still, chances are good that the drop of rain that splashes on your forehead is made of molecules that were here long before the first humans looked up in wonder at a cloudy sky, long before the first leafy plants stretched their roots into the soil, long before the first single-celled organisms took the critical step of dividing in half to reproduce.